Method and system for multichannel-isolation-technique multiplexer

ABSTRACT

A multiplexer is disclosed. The multliplexer comprises a first input and a first channel coupled to the first input. The multiplexer further includes a second input and a second channel coupled to the second input. Finally, the multiplexer includes an output coupled to the first and second channels, wherein a coupling capacitance of an inactive one of the first and second channels is not coupled directly to the output. A method and system in accordance with the present invention reduces crosstalk and jitter in a multiplexer by eliminating the coupling capacitance between an inactive input and the output. In so doing, there is significantly better isolation between channels thereby minimizing the aforementioned cross-talk and jitter.

FIELD OF THE INVENTION

The present invention relates generally to multiplexers and moreparticularly to a method and system for reducing cross-talk and jitterin such multiplexers.

BACKGROUND OF THE INVENTION

In conventional multiplexers the output voltage of the active output isinfluenced by the non-active input if both inputs have similar phaseconditions. The reason for this behavior is the coupling capacitance Ccbof the non-active input to the active output. This results in anon-optimal isolation of the active and the non-active channel withhigher crosstalk and higher jitter.

FIG. 1 is a diagram of a conventional multiplexer 10. The conventionalmultiplexer includes first and second differential amplifiers 12 and 14.As is seen, differential amplifier 12 comprises transistors 16 a and 16b, and differential amplifier 14 comprises transistors 18 a and 18 b.The conventional multiplexer 10 has its non-inverted Select input set tohigh and its inverted Select input set to low. The active channel isinput IN1 with the transistors 16 a, 16 b, 20 a and 20 b. Accordingly,there is a high coupling capacitance between the non-active input IN2and the active output. Therefore it is desirable to provide betterisolation between active and inactive channels and thereby reducecrosstalk and jitter.

Accordingly, what is needed is a system and method for overcoming theabove-identified problem. The present invention addresses such a need.

SUMMARY OF THE INVENTION

A multiplexer is disclosed. The multliplexer comprises a first input anda first channel coupled to the first input. The multiplexer furtherincludes a second input and a second channel coupled to the secondinput. Finally, the multiplexer includes an output coupled to the firstand second channels, wherein a coupling capacitance of an inactive oneof the first and second channels is not coupled directly to the output.

A method and system in accordance with the present invention reducescrosstalk and jitter in a multiplexer by eliminating the couplingcapacitance between an inactive input and the output. In so doing, thereis significantly better isolation between channels thereby minimizingthe aforementioned cross-talk and jitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional multiplexer.

FIG. 2 is a diagram of a multiplexer in accordance with the presentinvention.

DETAILED DESCRIPTION

The present invention relates generally to multiplexers and moreparticularly to a method and system for reducing cross-talk and jitterin such multiplexers. The following description is presented to enableone of ordinary skill in the art to make and use the invention and isprovided in the context of a patent application and its requirements.Various modifications to the preferred embodiment and the genericprinciples and features described herein will be readily apparent tothose skilled in the art. Thus, the present invention is not intended tobe limited to the embodiment shown but is to be accorded the widestscope consistent with the principles and features described herein.

A method and system in accordance with the present invention reducescrosstalk and jitter in a multiplexer by eliminating the couplingcapacitance between an inactive input and the output. In so doing, thereis significantly better isolation between channels thereby minimizingthe aforementioned cross-talk and jitter.

In a preferred embodiment, the coupling capacitance is moved one leveldown from the output level. Hence, there is no direct coupling betweenan active input and a non-active input to the output. This modificationin the present multiplexer reduces the crosstalk and jittersignificantly. To describe these features in more detail, please refernow to the following description in conjunction with the accompanyingfigure.

FIG. 2 is a diagram of a multiplexer 100 in accordance with the presentinvention. Please find below a glossary of terms related to FIG. 2.

Glossary of Terms

-   S non-inverted Select input-   Sb invertedSelect input-   IN1 non-inverted Data input 1-   IN1 b inverted Data input 1-   IN2 non-inverted Data input 2-   IN2 b inverted Data input 2-   Q non-inverted Output-   Qb inverted Output-   Ccb Collector base capacitance-   tph Phase relation between input 1 and input 2    Operation

It is assumed the active input is IN1 with the correspondingdifferential amplifier 101 which comprises transistors 102 and 104. Thecollectors of transistors 102 and 104 are coupled to differentialamplifier 103, which comprises transistors 106 and 108, and adifferential amplifier 105, which comprises transistors 110 and 112.

The non-active input IN2 is coupled to differential amplifier 107, whichcomprises transistors 114 and 116. The collectors of transistors 114 and116 are coupled to differential amplifier 109, comprising transistors118 and 120, and differential amplifier 111, which comprises transistor122 and 124. The collectors of transistors 108, 110, 120 and 122 arecoupled to VCC. The collectors of transistors 106 and 118 are coupledtogether to resistor 126 and form the inverted output Qb. The collectorsof transistors 112 and 124 are coupled together to resistor 128 and formthe non-inverted output Q.

Consider now the case when the non-inverted Select input is high and theinverted Select input is low. Therefore, transistors 106, 112, 120 and122 are on and transistors 108, 110, 118 and 124 are off. The input IN1is passed to the output since IN2 has no impact because the collectorsof transistors 120 and 122 are coupled to VCC.

Consider the case when the inverted Select input is high and thenon-inverted Select input is low. Therefore, transistors 108, 110, 118and 124 are on and 106, 112, 120 and 122 are off. The input IN2 ispassed to the output since IN1 has no impact because the collectors oftransistors 108 and 110 are coupled to VCC. It is assumed that there isa constant low impedance DC level of the inverted and non-invertedSelect input. Accordingly, there is a minimum impact of the non-activechannel on the active channel since the coupling capacitance of thenon-active channel is not coupled directly to the output.

A method and system in accordance with the present invention reducescrosstalk and jitter in a multiplexer by eliminating the couplingcapacitance between an inactive input and the output. In so doing, thereis significantly better isolation between channels thereby minimizingthe aforementioned cross-talk and jitter.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

1. A multiplexer comprising: a first input; a first channel coupled tothe first input; a second input; a second channel coupled to the secondinput; an output coupled to the first and second channels, wherein acoupling capacitance of an inactive one of the first and second channelsis not coupled directly to the output.
 2. The multiplexer of claim 1wherein the first channel comprises: a first input differentialamplifier (DAF) coupled to the first input; and a first plurality oftransistors coupled between the first DAF and the output.
 3. Themultiplexer of claim 2 wherein the second channel comprises: a secondinput differential amplifier (DAF) coupled to the second input; and asecond plurality of transistors coupled between the second DAF and theoutput.
 4. The multiplexer of claim 1 which includes a non-invertedselect input for activating the first channel and inactivating thesecond channel and an inverted input for inactivating the first channeland activating the second channel.
 5. The multiplexer of claim 3 whereinthe first plurality of transistors are turned off when the first channelis inactive.
 6. The multiplexer of claim 5 wherein the second pluralityof transistors are turned off when the second channel is inactive.